A new surveillance indicator identifying optimal timeliness and accuracy: application to the Korean National Notifiable Disease Surveillance System for 2001-2007.

Although immediate notification of a case is crucial for epidemic control, clinicians may delay notification due to uncertainties in diagnosis, reflecting a trade-off between timeliness and the accuracy of surveillance. We assessed this trade-off for four epidemic-prone diseases that require immediate notification of suspected cases: shigellosis, typhoid fever, paratyphoid fever, and cholera in the Korean National Notifiable Disease Surveillance System data for 2001-2007. Timeliness was measured as the time to registration (T R), being the time interval from symptom onset to notification by the clinician to the local public health centre. We introduced a new index, 'time-accuracy trade-off ratio' to indicate time saved by clinical vs. laboratory-based notifications. Clinical notifications comprised 34.4% of total notifications, and these showed a shorter median T R than laboratory-based notifications (1-4 days). The trade-off ratio was greatest for shigellosis (3.3 days), and smallest for typhoid fever (0.6 days). A higher trade-off ratio provides stronger evidence for clinical notification without waiting for laboratory confirmation.